if you just want a cage for the sake of having one (looks) then they're fine, if you want one for safety then stay away from the ones that bolt to existing points on the car, none of the ones ive seen have been much cop, including the msv one.
If it's just a road car then i'd not bother, as said above you'll just be adding weight :wink:
For track then the safety devices one's worth a look, or if you intend competing or track prep'ing the car then see if you can get a proper weld in cage or an ex cup racer cage (if you can find one)